PHYSIOLOGY OF THE DIGESTIVE GLANDS 435 



geal fistula. This experiment is known as that of "sham feeding." 

 Within a few minutes after giving food the gastric juice was found to 

 be secreted actively, and if the feeding process was kept up, which could 

 be done almost indefinitely since the animal never became satisfied, the 

 secretion continued to flow. Thus, in one instance Pavlov succeeded in 

 collecting about 700 c.c. of gastric juice after sham feeding an animal 

 for five or six hours in the manner above described. 



After the stomach has emptied itself of the food taken with the pre- 

 vious meal, it is said by Pavlov to contain only a little alkaline mucus. 

 The more recent work of Carlson, however, shows that this is not strictly 

 the case, there being more or less of a continuous secretion of gastric juice 

 in the entire absence of food. The amount varies from a few c.c. up to 

 60 c.c. per hour, more secretion being produced when it is collected every 

 five or ten minutes than if it is collected every thirty or sixty, thus 

 indicating that, ordinarily, some escapes through the pylorus into the 

 duodenum. The secretion contains both pepsin and hydrochloric acid. 

 As to the cause of this continuous secretion, little is known. It may be 

 an example of the periodic activities of the digestive glands described by 

 Boldyreff, or it may in part be due to a psychic stimulation dependent 

 upon the thought of food. That the latter is probably not the cause, is 

 indicated by the fact that, at least in Carlson's patient, the psychic juice 

 could not be made to flow short of giving food. 



The sham feeding causes stimulation of the gastric secretion through 

 impulses transmitted to the stomach along the vagus nerves; for it has 

 been found, in animals in which the vagus nerve has been cut, that the 

 sham feeding no longer induces a secretion of gastric juice. The ques- 

 tion therefore arises as to how the nerve center is stimulated. Three 

 possible causes may be considered: (1) mechanical stimulation of the 

 sensory nerves of the mouth; (2) chemical stimulation of the nerves; 

 (3) the agreeable stimulation of the taste buds and olfactory endings 

 concerned in the tasting of food. In investigating these possibilities, 

 mechanical stimulation was readily ruled out by showing that mere 

 taking of solid matter in the mouth did not excite any secretion, although 

 it might cause a flow of saliva. Mere chemical stimulation could not be 

 the cause, for no secretion was induced by placing substances such as 

 acetic acid or mustard oil in the mouth. By exclusion, then, it would 

 appear that the adequate stimulus must consist in the agreeable stimula- 

 tion of the taste buds, etc. that is to say, in the gratification of appetite. 



Further justification for this conclusion was readily secured by noting 

 that foodstuffs for which the animal had no particular desire or appe- 

 tite failed to excite the secretion. Most dogs, for example, although 

 they may take it, are not particularly fond of bread, and when fed with 



